The present invention relates to a system and method for suppressing electrical surges and transients on high frequency communication networks typified by CATV communication systems.
Electrical transients, such as those caused by electromagnetic interference (fluorescent lights, transformers, automobile ignitions, etc.), operation of electrical switches, short circuits, electrical storms and the like, cause disruption, false operation and even damage to electrical and electronic equipment connected to electrical power networks, and to communication systems and equipment. A variety of devices and circuits have been developed for suppressing electrical transients and surges and thus protecting electrical power systems and information transmission systems. Some of these devices include gas tubes (an electron tube containing a small amount of gas or vapor which ionizes when a certain voltage is developed across the tube, with such ionization allowing a greatly increased current flow to discharge the increasing voltage), metal oxide varistors (a two-electrode semiconductor device in which a metal oxide is placed between the electrodes so that as voltage across the metal oxide increases to a certain level, the metal oxide breaks down or becomes conductive and current is conducted through the metal oxide to "clamp" the voltage to said certain level), and avalanche diodes (a silicon diode that acts like a rectifier until applied voltage reaches a certain value, known as the avalanche reverse-breakdown voltage--typically more than eight volts, at which point the diode becomes conducting with the voltage drop across the diode remaining essentially constant independent of the current). Combinations of these devices have also been utilized, along with other electrical components, to form hybrid circuits for suppressing electrical transients.
Special problems arise in attempting to suppress electrical transients and surges in high frequency or radio frequency communication lines since most times the suppression of the transient also results in attenuation of the high frequency signals being transmitted over the lines. Such signal attenuation in, for example, a cable television (CATV) communication system, results in a deterioration of picture quality.
Conventional approaches at surge suppression in high frequency communication lines have generally involved the use of a single component, usually a gas tube, or a three-stage hybrid circuit such as shown in FIG. 1A. The FIG. 1 prior art circuit consists of a gas tube, a resistor or inductor and an avalanche diode or diodes. This circuit, although providing good "clamping" at a high surge current rating, also acts as a low-pass filter (the combination of the capacitance of the avalanche diode and the resistor [or inductor]) which attenuates high frequency signals. As a consequence, the circuit of FIG. 1A is suitable as a surge suppressor only for communication lines whose signal frequencies range up to about 10 MHz.
FIGS. 1B and 1C show other prior art circuits devised to improve the upper frequency cutoff level, both of which utilize low-capacitance switching diodes for reducing the effective capacitance of avalanche diodes, also utilized in both circuits. The circuits of FIGS. 1B and 1C are generally effective for communication lines whose signal frequencies range up to about 50 MHz.
The problems discussed above in devising a suitable surge suppression circuit for high frequency communication lines have been addressed by at least one commentator who stated that "protection of rf circuits was largely an unsolved problem" and then quoted Military Handbook 419 (1982, p. 1-84) to the effect that "effective suppression devices/circuits are not currently available for in-line installation on rf lines above 3 MHz primarily because of high insertion losses". Standler, Ronald B., "Protection of Electronic Circuits from Overvoltages", John Wiley and Sons, page 152.